The invention relates to a process for purifying aluminum contaminated with at least one of the metals of groups II b, III a, IV a and V a of the periodic table of elements by means of sodium and liquid extraction in a separating column in the temperature range spanning the melting point of aluminum or its alloys and the boiling point of sodium.
In the conventional process for producing aluminum via fused salt electrolysis in a cathodic tank lined with carbon and using carbon anodes the primary metal produced has a purity of 99.5% and more. In such a process neither the electrodes nor the molten electrolyte contaminate the molten aluminum to a significant degree.
Apart from the conventional process there exist specific processes for producing aluminum in which the metal is not produced in a relatively pure form but as an alloy:
The molten cathode of a fused salt reduction cell can be of lead or tin which combines with the precipitated aluminum to form a range of alloys. PA1 In the carbothermic process for producing aluminum lead or tin is employed as absorbing agent either directly in the reduction furnace or after this in order to enable the aluminum to be separated from silicon and iron. PA1 pre-heated sodium is introduced into the separating column from a circuit having a heat exchanger, solids precipitator, pump and storage tank, in quantities at least 5 times greater than the mass of contaminating metal, and flowing countercurrent to the contaminated aluminum which is also introduced into the separating column, PA1 the purified aluminum containing a small residual amount of contaminating metal and a sodium content not exceeding 500 ppm is removed and put to use, and PA1 the sodium containing a low concentration of contaminating metal is returned to the circuit.
These methods of manufacturing aluminum can be employed on a significant technical scale if it is made possible to remove the residual 0.8 to 1.5% lead and tin to a level beyond the eutectic point.
In the U.S. Pat. No. 2,239,277 a process for purifying molten aluminum or aluminum alloys is proposed and namely such containing up to 1% of at least one of the impurities: antimony, bismuth, lead and tin. This is achieved by adding 0.1 to 1% sodium to the melt and stirring. As a result the dross so formed can rise to the surface of the melt and be removed. The addition of sodium takes place preferably in accordance with the stoichiometric amount required to form an intermetallic compound with the metal in question. Using this method it is possible, for example, to lower the lead content of an aluminum alloy from 0.94% to 0.28%. This concentration, however, lies far in excess of the permitted 50 ppm limit for lead in aluminum, thus making the process inapplicable on an industrial scale.
Described in the German Patent Publication No. 24 38 064 is a process for purifying metallic aluminum--in particular for the removal and recovery of bismuth, cadmium, gallium, mercury and tin. These elements can be present in metallic aluminum in amounts which are unacceptable for aluminum of normal commercial purity. The molten aluminum is brought into contact with an alkali metal such as sodium or potassium, for example. As with the above mentioned U.S. Pat. No. 2,239,277 the alkali metal is added in an amount which corresponds with the stoichiometry for formation of an intermetallic compound.
The proposed process according to the DE-OS No. 24 38 064 is such that provision is made, upstream for a conventional counter-flow extraction reactor, for a mixing stage where aluminum with a high concentration of tin is mixed with sodium having a low tin content. Subsequently, likewise upstream of the counter-flow extraction reactor, the mixture is separated in a series of parallel settling columns into two layers viz., a layer of aluminum low in tin content and a tin-rich sodium layer. The density of these two layers varies depending on the tin content of the sodium layer and does so to such an extent that one or the other layer lies on top. This overlapping of densities can cause difficulties in practice. The tin-bearing sodium alloy leaving the counter-flow extractor is fed back to the mixing stage. The purified aluminum is freed of the excess sodium by distillation of the sodium. The removal of tin from the equipment takes place in the second stage where tin-rich sodium alloys are removed.
The separation and removal of sodium rich tin alloys at elevated temperatures i.e. above the melting point of aluminum involves a number of technical problems; furthermore, a lot of sodium is lost with the tin which is removed.
The equipment employed for the process according to the German Patent Publication DE-OS No. 24 38 064 is complicated and the process itself is energy intensive.
It is therefore an object of the present invention to develop a process for purifying contaminated aluminum of at least one of the metals of groups II b, III a, IV a and V a of the periodic table of elements and this by means of a simple device and low energy consumption.